Abstract: Based on the core observation, thin sections, SEM and X-ray diffraction combined with the reservoir properties analysis, the relationship between diagenesis and porosity evolution of middle Jurassic-lower Cretaceous reservoirs (Xishanyao Formation (J₂x), Toutunhe Formation (J₂t) and Qingshuihe Formation (K₁q))in Shinan area, Junggar basin have been researched in this paper. The results show that these reservoirs are mainly consist of fine sandstone, mid-fine grained sandstone and medium sandstone, with low composition maturity and medium structural maturity. The correlation of pore and permeability is good, and the throat is dominated by middle throat, with part of fine throat. These reservoirs belong to middle and low permeability reservoir on the whole. The reservoirs porosities are primarily composed of primary porosity, and the second porosity caused by dissolution is very rare. The Middle Jurassic-Lower Cretaceous reservoirs primarily are formed in A period of early diagenesis and B period of early diagenesis, according to the diagenetic characteristics and pore assemblage in Shinan area. The diagenetic sequence of the reservoirs primarily is chlorite precipitation and early calcite cementation, feldspar and lithic dissolution, early kaolin and siliceous cementation, secondary kaolinite precipitation and quartz overgrowth, late ankerite and ferro calcite cementation. The result shows that the compaction was occurred through the whole burial diagenetic process, and the middle Jurassic-lower Cretaceous reservoirs have mainly undergone a series of diagenesis, such as compaction, cementation and dissolution. The compaction is the main cause leading to the reduction of the reservoirs porosity, which followed by the cementation, with limited contribution of dissolution to reservoir pore increase.